Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride and less than 5 mole percent iodide, based on total silver.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of blue and minus blue imaging speeds, and improved image sharpness in both mono and multi emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
In general the greater the proportion of the total grain population accounted for by tabular grains, the greater the advantages realized. This parameter is typically specified in terms of the percentage of the total grain projected area accounted for by the selected tabular grain population.
The property of the selected tabular grain population which sets it apart from the remaining grains, if any, in the emulsion and predicts its advantages in relation to other selected tabular grain populations is herein referred to as "tabularity", where the mean tabularity of a selected tabular grain population is determined from the relationship: EQU D/t.sup.2
where
D is the effective circular diameter (ECD) in .mu.m of the tabular grains and PA0 t is the thickness in .mu.m of the tabular grains.
Although the art has succeeded in preparing high chloride tabular grain emulsions, the inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. The basic reason is that tabular grains are produced by incorporating parallel twin planes in grains grown under conditions favoring {111} crystal faces. The most prominent feature of tabular grains are their parallel {111} major crystal faces.
To produce successfully a high chloride tabular grain emulsion two obstacles must be overcome. First, the strong propensity of silver chloride to produce {100} crystal faces must be overcome by finding conditions that favor the formation of {111} crystal faces. Second, conditions must be found that incorporate parallel twin planes in the grains.
Wey U.S. Pat. No. 4,399,215 produced the first high aspect ratio (D/t&gt;8) silver chloride emulsion. An ammoniacal double Jet precipitation technique was employed. The tabularity of the emulsions was not high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia thickened the tabular grains. A further disadvantage was that significant reductions in tabularity occurred when bromide and/or iodide ions were included in the tabular grains.
Wey et al U.S. Pat. No. 4,414,306 developed a process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions.
Maskasky U.S. Pat. No. 4,400,463 developed a strategy for preparing a high chloride, high aspect ratio tabular grain emulsion capable of tolerating significant inclusions of the other halides. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces. Adsorbed aminoazaindenes and iodide ions were disclosed to be useful grain growth modifiers. This work has stimulated further investigations of grain growth modifiers for preparing tabular grain high chloride emulsions, as illustrated by Takada et al U.S. Pat. No. 4,783,398, which employs heterocycles containing a divalent sulfur ring atom; Tufano et al U.S. Pat. No. 4,804,621, which employs amino substituted diazines; and Nishikawa et al U.S. Pat. No. 4,952,491, which employs spectral sensitizing dyes during nucleation and divalent sulfur atom containing heterocycles and acyclic compounds during grain growth.
Maskasky U.S. Pat. No. 4,713,323, continuing to use an aminoazaindene growth modifier, discovered that tabular grain high chloride emulsions could be prepared by running silver salt into a dispersing medium containing at least a 0.5 molar concentration of chloride ion and an oxidized gelatino peptizer. An oxidized gelatino peptizer is a gelatino peptizer treated with a strong oxidizing agent to modify by oxidation (and eliminate or reduce as such) the methionine content of the peptizer. Maskasky taught to reduce the methionine content of the peptizer to a level of less than 30 micromoles per gram. King et al U.S. Pat. No. 4,942,120 is essentially cumulative, differing only in that methionine was modified by alkylation.
The discoveries that (1) strongly adsorbed grain growth modifiers can be used to achieve {111} crystal faces during the precipitation of high chloride emulsions and (2) chloride ion concentrations above 0.5M can be used to induce twin planes in the high chloride grains have provided the capability of preparing high chloride tabular grain emulsions. There has remained, however, the problem that the strongly adsorbed grain growth modifiers not only occupy grain surface sites as the grains are being formed, but also remain after grain formation. This places the adsorbed grain growth modifiers in competition with a wide variety of conventional emulsion addenda (such as chemical and spectral sensitizers, antifoggants and stabilizers, nucleating agents, etc.) that require grain adsorption to be effective.
This has led those skilled in the art to search for alternative choices in grain growth modifiers. K. Endo and M. Okaji, "An Empirical Rule to Modify the Crystal Habit of Silver Chloride to Form Tabular Grains in an Emulsion", J. Photographic Science, 1988, Vol. 36, (1988), pp. 182-189, set out to produce an empirical rule for selecting materials for use as grain growth modifiers in preparing silver chloride tabular grain emulsions by double-jet precipitation. The rule was tested by adding various ligands, CN.sup.-, SCN.sup.-, I.sup.-, (S.sub.2 O.sub.3).sup.-2, (SO.sub.3).sup.-3 and thiourea (including derivatives) to 3M sodium chloride solutions at concentrations of 0.001, 0.005, 0.01 and 0.1M. The 3M sodium chloride solution was then used with 2M silver nitrate in double jet precipitations. Tabular grains having {100} and {111} faces were produced. Based on these investigations Endo et al concluded that to be useful as a grain growth modifier in forming tabular grain high chloride emulsions the first formation constant of the ligand, .beta..sub.1 (L), must be more than .beta..sub.2 (Cl.sup.-)--i.e., .beta..sub.2 (Cl.sup.-)/.beta..sub.1 (L) must be less than unity (one). In Table 2 Endo et al reported .beta..sub.2 (Cl.sup.-)/.beta..sub.1 (L) for SCN.sup.- to be 6.3, thereby indicating SCN.sup.- not to be suitable for use as a grain growth modifier. In FIG. 7 Endo et al shows a silver chloride grain population produced using 0.10M KSCN. The grains are relatively thick and are bounded by {100} top and bottom crystal faces, as is evident from the observed right angle crystal face intersections.
Although Endo et al rejected SCN.sup.- as a useful grain growth modifier in forming tabular grain high chloride emulsions, considering the known compatibility of thiocyanate ion with high levels of photographic performance it is not surprising that thiocyanate ions were among the candidates considered. Alkali metal and ammonium thiocyanates have been used for many years in silver halide photography as ripening agents both during and following the grain precipitation step. Nietz and Russell U.S. Pat. No. 2,222,264 report the single jet precipitation of silver chloride in the presence of thiocyanate. Kofron et al U.S. Pat. No. 4,439,520 taught the use of thiocyanate as a ripening agent in the preparation of high aspect ratio tabular grain emulsions and also in their sensitization.